1. Field of the Invention
The present invention relates to a signal measuring apparatus and a signal measuring method in which a high speed actuating signal generated in a semiconductor large scale integrated circuit (hereinafter, called LSI) or a semiconductor very large scale integrated circuit (hereinafter, called VLSI) is measured.
2. Description of the Prior Art
Recently, an LSI or VLSI having a very fine circuit pattern has been manufactured with the development of a fine processing technique resulting from the advance of a mask and exposing technique. Therefore, an inspection technique for inspecting the very fine circuit pattern has been advanced to a high level from year to year, and the enhancement of performances such as a spatial resolution and a time resolution in an LSI inspection apparatus or a failure analysis apparatus has been requested more and more strictly.
In an inspection apparatus in which a conventionally used laser probe or an electron-beam probe is utilized, restrictions resulting from a measuring principle such as a scale-down limitation of a probe diameter resulting from a restriction of a light wavelength and a disturbance of an accurate voltage measurement resulting from a charge-up of samples exist as problems. In the future, in cases where a very fine circuit pattern manufactured with the development of a fine processing technique is inspected with a conventional inspection apparatus, the inspection of the fine circuit pattern becomes more and more difficult.
Therefore, a signal measuring apparatus and a probing method in which a high speed actuating signal generated in a semiconductor device can be measured with a scanning tunnel microscope (STM) or an atomic-force microscope (AFM) in which an atomic image can be observed is required.
Hereinafter, a signal measuring method in which an actuating signal generated in an LSI or VLSI having a very fine circuit pattern is measured is described.
FIG. 1 is a constitutional view of a non-contact probe type of LSI inspection apparatus (hereinafter, called a first apparatus) in which a scanning tunnel microscope (STM) disclosed in a literature (30a-P-4) of Society for Applied Physics in autumn of 1993. In FIG. 1, a piezo actuator 1A has a probe 101, a piezo device 102 and a feed back actuating element 103. In the piezo actuator 1A, a distance between the probe 101 and a device 26 is adjusted to get a constant value of tunnel current I.sub.t between the probe 101 and the device surface 26.
2A denotes a photoconductive element for chopping the tunnel current I.sub.t. When a laser beam is radiated to the photoconductive element 2A, the photoconductive element 2A is excited and is instantaneously set in a conductive condition. The photoconductive element 2A is placed between the probe 101 and the piezo element 102. 3A denotes a waveform measuring circuit for converting the tunnel current I.sub.t into a voltage and measuring a waveform of the voltage, and the waveform measuring circuit 3A has a current/voltage converter (hereinafter, simply called I/V converter) 301 and an amplifier and low pass filter 302.
4A denotes a pulse oscillator (or a pulse generator) for converting a sine wave signal having a frequency f+.DELTA.f into a trigger signal and generating a series of pulses having the frequency f+.DELTA.f to sample the tunnel current I.sub.t. In this case, the frequency f denotes an actuating frequency of the device 26 and is equal to almost 100 MHz. An intensity of the tunnel current I.sub.t is changed by an actuating signal of the device 26. The symbol .DELTA.f denotes a differential frequency and is equal to about several KHz. 5A denotes a laser diode for generating a laser beam according to the series of pulses having the frequency f+.DELTA.f.
6A denotes a function generator for outputting the sine wave signal having the frequency f+.DELTA.f to the pulse generator 4A, 7A denotes a mixing circuit for mixing the sine wave signal having the frequency f+.DELTA.f transferred from the function generator 6A and a sine wave signal having a frequency f and forming a beat signal having the frequency .DELTA.f, 8A denotes a function generator for outputting the sine wave signal having the frequency f to the device 26 and the mixing circuit 7A, and 9A denotes an oscilloscope for sampling an output voltage of the amplifier and low pass filter 302 in synchronism with the beat signal having the frequency .DELTA.f and displaying a waveform of the output voltage sampled.
Next, an operation of the first apparatus is described. For example, in cases where a waveform of voltage of a strip line (hereinafter, simply called a device) 16 in a 50 L system is observed as a sample, a position of the probe 101 is adjusted by the piezo actuator 102 to approach to a wiring of the device 26 as close as possible for the purpose of passing the tunnel current I.sub.t between the probe 101 and the device 26. The tunnel current I.sub.t passes through a route of the I/V converter 301.fwdarw.a ground GND.fwdarw.the device 26.fwdarw.the probe 101.fwdarw.the photoconductive element 2A.fwdarw.the I/V converter 301. In this case, the distance between the probe 101 and the device 26 is controlled to a constant value by the piezo actuator 102 which is operated under the control of the feed back actuating element 103 according to the tunnel current I.sub.t.
Thereafter, to operate the device 26, a sine wave signal (or a clock signal) having a frequency f is supplied from the function generator 8A to the device 26 through a Schottky diode. While supplying the sine wave signal having the frequency f, a trigger signal having a frequency f+.DELTA.f, which differs from the frequency f by a differential frequency .DELTA.f, is generated in the function generator 6A, and the trigger signal is output to the pulse oscillator 4A and the mixing circuit 7A. In the pulse oscillator 4A, a series of pulses having the frequency f+.DELTA.f is generated according to the trigger signal, and a laser beam L is generated according to the series of pulses having the frequency f+.DELTA.f in the laser diode 5A. Thereafter, when the laser beam L is radiated to the photoconductive element 2A, the photoconductive element 2A is instantaneously excited to a conductive state because the photoconductive element 2A is excited by the laser beam L. As a result, the tunnel current I.sub.t flowing between the prove 101 and the device 26 is chopped. The tunnel current I.sub.t varies with an actuating voltage of the device 26. The tunnel current I.sub.t is converted into a voltage in the I/V converter 301, subsequently the voltage is amplified and filtered in the amplifier and low pass filter 302 to form an output voltage, and the output voltage shaped is input to the oscilloscope 9A.
Also, while supplying the sine wave signal having the frequency f, a sine wave signal having the frequency f transferred from the function generator 8A and the trigger signal having the frequency f+.DELTA.f transferred from the function generator 6A are mixed in the mixing circuit 7A to form a beat signal having a frequency .DELTA.f, and the beat signal is output to the oscilloscope 9A. As a result, in the oscilloscope 9A, the output voltage transferred from the amplifier and low pass filter 302 is sampled in synchronization with the beat signal. Therefore, a high speed waveform of voltage transferred from the device 26 is observed, and a relative electric potential of the device 26 is found out.
FIG. 2 is a constitutional view of a contact probe type of LSI inspection apparatus (hereinafter, called a second apparatus) in which an atomic force microscope is applied. In FIG. 2, the second apparatus is provided with a piezo actuator 1B, a photoconductive element 2B, a waveform measuring circuit 3B, a pulse oscillator (or a pulse generator) 4A, a laser diode 5B, two function generators 6B, 8B, a mixing circuit 7B and an oscilloscope 9B. The piezo actuator 1B is composed of a contact probe 104, piezo element 105, a feed back actuating element 106 and a photo-detector 107, and a photoconductive element 2B is arranged between the contact probe 104 and the piezo element 105. The contact probe 104 is attached to an AFM cantilever. The waveform measuring circuit 3B has a current/voltage converter (hereinafter, simply called I/V converter) 303 and an amplifier and low pass filter 304. The elements of which names are the same as those in the first apparatus have the same functions as those in the first apparatus. Therefore, the description of the functions of the elements which are the same as those in the first apparatus are omitted.
Next, an operation of the second apparatus is described. For example, in cases where a waveform of voltage in the device 16 which is not covered with any insulating film, the contact probe 104 is set in ohmic-contact with the device 16 by the piezo element 105. This ohmic-contact condition is controlled to a constant contact using a principle of a laser leverage. For example, the feed-back actuating element 106 functions according to a displacement detecting signal detected by the photo-detector 107, and,the contact probe 104 is in constant contact with the device 16 by the piezo element 105 under the control of the feed back actuating element 106. Thereafter, a sine wave having a frequency f is supplied from the function generator 8B to the device 16 through a diode, in the same manner as the first apparatus.
While supplying the sine wave having the frequency f, a trigger signal having a frequency f+.DELTA.f which differs from the frequency f of the sine wave by a differential frequency .DELTA.f is generated in the function generator 6B, and the trigger signal is output to the pulse oscillator 4A and the mixing circuit 7B. In the pulse oscillator 4A, a series of pulses having a frequency f+.DELTA.f is generated according to the trigger signal, and a laser beam L is generated in the laser diode 5B according to the series of pulses having the frequency f+.DELTA.f. When the laser beam L is radiated to the photoconductive element 2B, the photoconductive element 2B is instantaneously set in a conductive condition because the photoconductive element 2B is excited by the laser beam. Therefore, an ohmic current I flowing between the probe 104 and the device 16 is chopped. The ohmic current I varies with an actuating voltage of the device 16. The ohmic current I is converted into a voltage in the I/V converter 303, the voltage is shaped in the amplifier and low pass filter 304 to form an output voltage, and the output voltage is output to the oscilloscope 9B.
Also, while supplying the sine wave signal having the frequency f, the sine wave signal having the frequency f transferred from the function generator 8B and the trigger signal having the frequency f+.DELTA.f transferred from the function generator 6B are mixed in the mixing circuit 7B to form a beat signal having a frequency .DELTA.f, and the beat signal is output to the oscilloscope 9B. As a result, in the oscilloscope 9B, the output voltage transferred from the amplifier and low pass filter 302 is sampled in synchronization with the beat signal. Therefore, a high speed waveform of voltage transferred from the device 16 is observed, and a relative electric potential of the device 16 is determined.
In the first apparatus described as an example of the prior art, an input impedance of the probe 101 is set to a high value, so that the tunnel current I.sub.t is set to several tens of nano amperes. Therefore, a circuit load in the device 26 can be reduced. Also, because a voltage of the device 26 can be measured in a perfect non-contact and non-destructive condition, a spatial resolution can be enhanced. Here, the spatial resolution denotes an ability for separating two signals having frequencies close to each other.
However, in the first apparatus, the waveform measuring circuit 3A functions to process the tunnel current I.sub.t having a very low value. Therefore, in cases where an actuating signal in the device 26 has an amplitude higher than a prescribed value, an energy level of the actuating signal exceeds a tunnel region, and there is a problem that it becomes difficult to measure the voltage of the device 26 in a superior linearity. An upper limit of the voltage is about 1 Vp-p at the most. Also, there is another problem that it is difficult to obtain a surface image of the device 26 and the waveform of the voltage at the same time.
In the second apparatus, the contact probe 104 can be stably in contact with the device 16 using the cantilever in which a laser leverage method is utilized, and a surface image of the device 16 can be obtained in addition to the stable contact. In addition, because the contact probe 104 is in directly ohmic-contact with the device 16, even though the actuating voltage of the device 16 is high, the voltage can be measured in a superior linearity.
However, in the second apparatus, because an ohmic contact resistance is comparatively low when the photoconductive element 2B is set in an "on" condition, the ohmic contact resistance undesirably functions as a circuit load in the device 16. Also, in cases where an insulating layer such as an air-oxide film or the like remains on the device 16, it is required to set the contact probe 104 in ohmic contact with a wiring of the device 16 while the contact probe 104 breaks through the insulating layer. In this case, it is required push the contact probe 104 onto the device 16. Therefore, there is a probability that a high sharp needle mint of the contact probe 104 is damaged and broken.
Also, in the second apparatus, because the cantilever having a low spring constant is used, it is difficult to push down the cantilever with a sufficiently strong force. In addition, the ohmic current flowing through the contact probe 104 is determined by the ohmic contact resistance and an input impedance of the I/V converter 303. Therefore, in cases where a metallic surface of a top portion of the contact probe 104, a wiring portion of the device (or sample) 16 or the like is covered with an oxide film, the ohmic contact resistance varies. In this case, there is a problem that it is difficult to probe the device 16 with a high reliability.